`round_to` should be an optional kwarg in `RegressionDiscontinuity.summary()`

For all the other experiment classes the function signature is

```python
def summary(self, round_to=None) -> None:
```

but for `RegressionDiscontinuity` it is

```python
def summary(self, round_to: None) -> None:
```

So there is an error in this function signature.